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2017 G80 3.8L Rough Idle and Check Engine Light - Misfire Code P0303

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Mike T. · rep 0 · May 25, 4:35 AM · 1 replies · solved · Question

Vehicle: 2017 Genesis G80 3.8L Premium

Hey everyone,

My 2017 Genesis G80 with the 3.8L engine has developed a really rough idle lately. It feels like the engine is shaking more than usual, especially when I'm stopped at a light. The check engine light came on yesterday, and when I scanned it, I got a P0303 code, indicating a misfire on cylinder 3.

I've noticed a slight hesitation when accelerating from a stop too. The car has about 75,000 miles on it. I'm wondering if this is a common issue for this model year and engine. Any ideas on what I should check first? Could it be spark plugs, ignition coils, or something more serious?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 4:35 AM

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ClearTheCode admin answer:

The P0303 diagnostic trouble code (DTC) specifically indicates a misfire detected in cylinder #3. This is a common issue that can stem from several sources in the 3.8L GDI engine. Addressing it promptly is important to prevent potential damage to the catalytic converter.

Here are the most common causes and diagnostic steps:

  1. Faulty Spark Plug: Worn or fouled spark plugs are a very frequent cause of misfires. If your plugs haven't been replaced recently (typically recommended around 60,000-75,000 miles for this engine), this is a good place to start.
  2. Defective Ignition Coil: The ignition coil for cylinder #3 might be failing. A simple test is to swap the coil from cylinder #3 with a known good cylinder (e.g., cylinder #1) and see if the misfire code follows the coil. If it does, the coil is the culprit.
  3. Clogged or Failing Fuel Injector: A fuel injector that isn't delivering enough fuel, or is stuck open/closed, can cause a misfire. This is less common than plugs or coils but is definitely a possibility. Fuel system cleaner can sometimes help, but a failing injector usually requires replacement.
  4. Compression Issue: In rare cases, a misfire can be due to low compression in the cylinder, indicating an internal engine problem like a bad valve or piston ring. This is usually accompanied by other symptoms like excessive oil consumption or smoke.
  5. Vacuum Leaks: While less likely to pinpoint a single cylinder, a significant vacuum leak could affect engine performance and potentially lead to misfires across multiple cylinders.

When to See a Shop:

  • If swapping coils doesn't move the misfire, and replacing the spark plug doesn't resolve it, a professional diagnosis is recommended. They can perform fuel injector tests, compression tests, and further advanced diagnostics to pinpoint the exact cause.
  • If the misfire is severe, causing significant shaking or the engine to stall, avoid driving the vehicle and have it towed to a reputable Genesis service center. Continuous driving with a severe misfire can quickly damage the catalytic converter, which is an expensive repair.
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